JPS5937726B2 - Cooling method for continuous annealing steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cooling a continuously annealed steel strip, which is used to heat and soak a copper strip to a temperature higher than its recrystallization temperature when producing a soft cold-rolled steel sheet with good drawability in a continuous annealing facility. The objective is to obtain a method that can obtain a product material with excellent drawability while efficiently cooling the product using relatively simple equipment and methods after overaging treatment for post-solid solution carbon precipitation.

When manufacturing soft cold-rolled steel sheets with good drawability using continuous annealing equipment, the cold-rolled copper strip is heated and soaked above the recrystallization temperature, and then over-aged to precipitate solid solution carbon. However, regarding the cooling of the copper strip after this over-aging treatment, there has been no suitable cooling technology that can increase the cooling rate, increase the cooling efficiency, and reduce the energy cost.

In other words, due to the restriction that the steel strip inside the furnace should not be oxidized, the reducing atmospheric gas (usually H2 is 3 to 7% and N2
The method involves circulating N2-N2 gas (with a concentration of 93 to 97%), cooling it with a cooler, increasing the pressure with a blower, and spraying the gas onto the copper strip from an injection nozzle installed in the furnace to cool it. In this system, as shown in FIG. 1, there are not only a large number of spray nozzles 31 along the copper strip pass line, but also a cooler (usually composed of a refrigerator 32 and a cooling exchanger 33) for cooling the circulating gas. A huge circulation fan 34 and a circulation duct 35 are required to circulate this gas, and since the cooling efficiency is low (g), a long copper strip path is required, so the furnace length is not long and the equipment cost increases. .

Moreover, the cost is unavoidably high because electric power, refrigerant gas, and a large amount of cooling water are required to operate the refrigerator and circulation fan as described above.

The present invention has been devised after repeated studies in view of the above circumstances, and the embodiment thereof is shown in the attached drawings. As shown in Figure 2, after being heated above the recrystallization temperature and soaked, it is once cooled to about 400°C, and at that temperature it is subjected to an overaging treatment for precipitation of solid solution carbon.
Mating for ~5 minutes and then cooling. During cooling after such overaging treatment, a metal rotating body with a coolant such as water passed through the inside is used, and a copper strip is attached to this metal rotating body. This method aims at cooling by bringing the materials into direct contact with each other.

By the way, regarding solid solute carbon in such a case, as shown in the equilibrium phase diagram in Figure 3, when cooling from a high temperature, the solid solute carbon changes depending on the cooling rate, for example. 400℃ to 200℃/5e(3)
In the case of high-speed cooling (like the 0 line), the amount of supersaturation is large, and if cooling is performed slowly along the equilibrium amount, the amount of supersaturation in the case of the 0 curve due to the above-mentioned high speed is as shown in line (B). This amount is six times that of slow cooling.

Moreover, the amount of residual solid solute carbon affects the quality of the steel sheet obtained, and steel sheets with a large amount of solute carbon have poor softness, as shown in Figure 4, which shows an example of rimmed steel. As a result, the elongation decreases, and as shown in FIG. 5, the aging property also deteriorates.

To put this specifically, C: 0.05%z Mn
In the case of rimmed steel with N: 0.3% and N: 0.0018%, the elongation when cooled from 400°C at 200'C/sec is 41%, but when cooled from 400°C to 30°C/sec, the elongation is 41%.
The elongation after cooling for 20 seconds was 46%, and there was a difference of 3 kg/- in terms of aging property.

Therefore, in the present invention, during cooling after overaging treatment for precipitation of solid solution carbon as described above, the cooling rate is set relatively at the beginning from the start to the end of cooling so that no supersaturated carbon remains during the cooling process. After that, cooling is performed using a water-cooled roll to gradually increase the cooling rate.

If the cooling rate is slow when starting cooling from point A as shown in Figure 3 with the lid on, the solid solution (
C) When the amount decreases and is slightly faster, the curve reaches 6 points, and when the cooling rate is very fast, there is no margin for precipitation time and solid solution (0 amount remains almost as it is and D・It looks like a dot.

According to the results of detailed study by the present inventors, in mild steel sheets with a carbon content of 0.03% to 0.08%, the cooling rate, the amount of solid solution (C) remaining, and the deterioration of mechanical properties, etc. In order to obtain a high-quality mild steel plate, the steel plate temperature (y), solid solution (C) amount (y in xi) should be
It turns out that the cooling should be carried out along the line of 2K Get results.

As an example, C: 0.05%, Mn: 0.3%,
Rimmed steel with 18% N:O,0O was heated at 700℃ in a continuous annealing furnace.
After heating and soaking to 400℃ to 30060℃. ,/sec
The steel strip was rapidly cooled at a cooling rate of If the material is cooled quickly, the material deterioration will be within 3% compared to the material obtained using the ideal method with a slow cooling rate.

Therefore, the present invention provides an efficient and highly efficient cooling method that increases the cooling rate step by step during the cooling process.
), that is, d: Copper strip thickness t: Metal rotating body shell thickness λ Thermal conductivity of the steel ΔTm: Average temperature difference between the copper strip and the cooling water Natural natural logarithm γ: Copper strip specific gravity Cp: Copper strip specific heat h Inside the steel and cooling water heat transfer coefficient υ: water flow rate β: arbitrarily change the constant determined by cooling water temperature, viscosity, etc.

Specifically, changing the shell thickness (1) of the metal rotating body, the thermal conductivity (λ) of the shell, or the water temperature and water volume by adjusting the water temperature and water volume lacks stability that restricts the cooling rate. Since the control method becomes complicated, the first step is to make a difference in shell thickness or shell heat conduction.

The manner in which the internal water-cooled metal rotating bodies are installed as described above is as shown in FIG. 3 to 8, the steel strip 9 is 180 mpm%, the diameter of the rotating body is 1 m, and the contact angle of the rotating body 3 with the steel strip 9 is 100°.
, rotating body 4,5°6.7 is 200°, rotating body 8 is 190°
°.

The amount of cooling water is determined by rotating body 3. is 5rn7'hr1 rotating body 4 is 7
77L/'hr s 10 m''/hr with rotating body 5
The speed for the N rotating body 6 was 13 m/hr, and the speed for the 1 rotating body T was 1777 i'/hr.

The details of the cooling mechanism are as shown in FIGS. 7 and 8, and the cooling water is supplied from the cooling water inlet pipe 10 through the flow rate adjustment valve 11 and the inlet rotary joint 12 to the metal rotating body 14 provided in the furnace wall 13, respectively. Cooling water is not supplied, and the cooled water is sent from the outlet rotary joint 15 to the discharge pipe 16, but the heated water in the pipe 16 becomes hot water and cannot be used for heat utilization in other departments. be done.

The internal structure of the metal rotating body 14 is as shown in FIG.
2 is provided, and the cooling water flowing in from the inlet joint 12 flows through the water channel 21 formed between the pitches, and in that direction, it is made to flow in the opposite direction to the steel strip 9 direction in order to increase the countercurrent relative velocity. , and was led to the piping 16 via the surface joint 15.

Specifically, the cooling rate performed with the above configuration and conditions is 30)/sec for the rotating body 3 in FIG.
Then, 34.5°c/5ec1 40° for rotating body 5
C/Sec, 48°0. /Sec, rotating body γ is 60°C/5ec1 rotating body 8 is 80°C/sec
c, and by performing cooling with the cooling rate gradually increased in this way, the cooling after the solid solution carbon precipitation treatment of the soft steel plate is made such that no supersaturated carbon remains in the cooling process, and moreover, it is rapidly cooled for comparison. Due to the compact equipment, favorable operability, and low operating costs, we were able to obtain products of excellent quality.

According to the present invention as described above, the steel strip after overaging treatment can be efficiently cooled in a short time using an efficient cooling method, and good quality can be obtained.
Moreover, it consumes less electricity and cooling water, and has excellent operability.
The equipment is relatively simple and compact, and the invention has many effects such as achieving the intended purpose at low cost, and is an industrially highly effective invention.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and FIG. 1 is an explanatory diagram of a cooling method according to the prior art, FIG. 2 is an explanatory diagram of a thermal cycle for manufacturing soft steel plate to which the present invention is applied, and FIG. is a diagram explaining the equilibrium state of solid solute carbon in steel, Figure 4 is a diagram showing the relationship between the amount of residual solid solute carbon in a steel plate and material elongation, and Figure 5 is a diagram showing the relationship with the amount of aging deterioration. FIG. 6 is an explanatory diagram of the device configuration of the cooling unit according to the present invention, FIG. 7 is an explanatory diagram of the cooling medium passage relationship, and FIG. 8 is a sectional view of the metal rotating body. However, in these drawings, 3 to 8 are metal rotating bodies 14.
11 is a flow regulating valve, 20 is a metal shell thereof, 21 is a refrigerant flow path, and 22 is a spiral partition plate.

Claims (1)

[Claims] 1. When cooling a continuously annealed copper strip, after performing over-aging treatment for precipitation of solid solution carbon, cooling is started slowly along the equilibrium amount, and from this cooling start, the cooling rate is gradually increased in multiple stages. A method for cooling a continuous annealed steel strip, characterized by continuous cooling.